John Thayer converted to the Catholic Church in 1783. Below is an excerpt from a letter to his brother explaining his reasons for converting.
That the Church May Be ONE
This indivisible unity of faith is evidently pointed out in Scripture, and Jesus Christ has established it as the foundationto the whole edifice, when He established His Church. She is one body, says St. Paul to the Ephesians (4:4-5), and we recognize but one Spirit which animates it, but one and the same Lord, but one and the same faith, but one baptism; that is to say, that our faith ought to be one in the same sense that Our Lord Jesus Christ is one. Now, Our Lord Jesus Christ is absolutely and essentially one; our faith, therefore, ought t be absolutely and rigorously one. In the prayer which Jesus Christ addressed to his Father for those who believe in him, he prayes that they may be united in faith, and that their union may resemble that of the three Persons of the Trinity, a union which he exhibits as a mark by which the world will infallibly know that He has received his mission from his Father. (John 17:20-21). Without this perfect union the world would never have been able to believe that he was sent by God.
You will perhaps suppose that Jesus Christ prayed at that same time that his disciples might be united in heart, and that he gives this mutual union formed and kept up by charity, as the distinctive mark of the society of Christians without any mention of unity of faith. But this very explanation proves the neccessity of being united in faith, since nothing destroys charity so much as a difference of religion. Witness all the disturbances and all the wars which the history of the different Sects records. Our Church alone can lay claim to this union, as an incontestable truth, and there needs no other proof to convince every mind not prepossessed against the Catholic Church that she alone is the true spouse of Jesus Christ.
All the Fathers of the Church unanimously insist on the necessity of a union in faith in the Church of Jesus Christ. You believe as we do that they are Saints. Now, taking into view the time when they lived, and how near they were to the time of Jesus Christ and His apostles, surely they could not be mistaken in this matter. I might produce in proof of what I here advance; an infinity of passages from their works, but my limits prevent me.